1. Field of the Invention
The present invention relates to a source follower, and more particularly, to a source follower capable of compensating the threshold voltage.
2. Description of the Related Art
After the liquid crystal material is first discovered by the European, the American dedicated in researching and developing its applications, the Japanese further deeply studies its physical characteristics and the applied technologies in various fields, so that new generation LCD (Liquid Crystal Display) has been continuously developed. Currently, various liquid crystal techniques had been widely applied in the display. Especially for the LCD, the TN-LCD (Twisted Nematic-Liquid Crystal Display) is expanded to the STN-LCD (Super Twisted Nematic-Liquid Crystal Display), and further expanded to the TFT-LCD (Thin Film Transistor LCD) by the makers, and its scale is getting bigger now. Further, some LCD makers start to develop the manufacturing techniques of the LTPS-LCD (Low Temperature Poly-Si Liquid Crystal Display) now.
Currently, the technique of the LTPS TFT (Low Temperature Poly-Si Thin Film Transistor) had been applied in the LCD panel, besides using as the active element of the LCD, it is also used to integrate the high integrated circuit into the circuit of the glass substrate. Wherein, the data line driving circuit is the portion that is rather hard to deal with, since the threshold voltage of the LTPS TFT is rather higher, if the amplifier circuit is used, the analog output voltage is impacted by the threshold voltage and thus results in the signal distortion.
Referring to both FIG. 1A and FIG. 1B, it schematically shows a circuit diagram and a timing diagram of a push-pull analog buffer capable of compensating the threshold voltage, respectively. The push-pull analog buffer 100 comprises an n-type transistor 110, a p-type transistor 120, a 1st switch 132, a 2nd switch 142, and a 3rd switch 152.
The drain 112 of the n-type transistor 110 is coupled to a positive voltage, and the source 114 of the n-type transistor 110 outputs an output voltage. The source 124 of the p-type transistor 120 is coupled to the source 114 of the n-type transistor 110, the drain 122 of the p-type transistor 120 is grounded, and the gate 126 of the p-type transistor 120 is coupled to the gate 116 of the n-type transistor 110. The 1st switch 132 comprises a 1st electrode 134 and a 2nd electrode 136. Wherein, the 1st electrode 134 is coupled to an input voltage, the 2nd electrode 136 is coupled to the gate 116 of the n-type transistor 110. The 2nd switch 142 comprises a 3rd electrode 144 and a 4th electrode 146. Wherein, the 3rd electrode 144 is coupled to an input voltage. The 3rd switch 152 comprises a 5th electrode 154 and a 6th electrode 156. Wherein, the 5th electrode 154 is coupled to the 4th electrode 146, and the 6th electrode 156 is coupled to the source 114 of the n-type transistor 110. The storage capacitor 162 comprises a 7th electrode 164 and an 8th electrode 166. Wherein, the 7th electrode 164 is coupled to the 2nd electrode 136, and the 8th electrode 166 is coupled to the 4th electrode 146.
The operating process of the push-pull analog buffer 100 is described hereinafter. At first, the 1st switch 132 and the 3rd switch 152 are shorted, and the 2nd switch 142 is opened. Meanwhile, the potential stored in the storage capacitor 162 is equal to the threshold voltage of the n-type transistor 110. Then, the 1st switch 132 and the 3rd switch 152 are opened, and the 2nd switch 142 is shorted. Meanwhile, the voltage at the gate 116 of the n-type transistor 110 is a summation of the input voltage and the threshold voltage, so that the output voltage is nearly equal to the input voltage. Wherein, the input, voltage is greater than the threshold voltage of the n-type transistor 110.
However, in the push-pull analog buffer 100, when the input voltage is smaller than the threshold voltage of the n-type transistor 110 and the p-type transistor 120, since the n-type transistor 110 and the p-type transistor 120 are working in the cut-off section, the push-pull analog buffer 100 cannot work normally. Further, when the input voltage is increased to reach the threshold voltage, the operating point of the push-pull analog buffer 100 is shifted, and there is some minor error existed in the output voltage and the input voltage. Therefore, the error value is very possible over the gray-level error tolerance when it is operated in high-level analysis.
In summary, the conventional source follower has following disadvantages: (1) In the push-pull analog buffer 100, the circuit cannot work normally when the input voltage is smaller than the threshold voltage of the transistor. (2) In the push-pull analog buffer 100, the error value is very possible over the gray-level error tolerance when it is operated in high-level analysis.